Methods and compositions for treating urinary incontinence using optically pure (S)-oxybutynin

ABSTRACT

A method for treating urinary incontinence while avoiding concomitant liability of adverse effects associated with racemic oxybutynin is disclosed. The method comprises administering a therapeutically effective amount of (S)-oxybutynin or a pharmaceutically acceptable salt thereof, substantially free of its R enantiomer. Pharmaceutical compositions comprising (S)-oxybutynin and an acceptable carrier are also disclosed.

FIELD OF THE INVENTION

The invention relates to method for treating urinary incontinence using optically pure (S)-oxybutynin and to pharmaceutical compositions containing optically pure (S)-oxybutynin.

BACKGROUND OF THE INVENTION

Racemic oxybutynin is used therapeutically in the treatment of intestinal hypermotility and in the treatment of urinary incontinence due to detrusor instability. Racemic oxybutynin exerts a direct antispasmodic effect on smooth muscle and inhibits the action of acetylcholine on smooth muscle. It exhibits only one-fifth of the anticholinergic activity of atropine on the rabbit detrusor muscle, but four to ten times the antispasmodic activity. It is quite selective for muscarinic receptors in the presence of nicotinic receptors and as a result, no blocking effects are observed at skeletal neuromuscular junctions or autonomic ganglia.

Racemic oxybutynin relaxes bladder smooth muscle and, in patients with conditions characterized by involuntary bladder contractions, cystometric studies have demonstrated that racemic oxybutynin increases vesicle capacity, diminishes the frequency of involuntary contractions of the detrusor muscle, and delays the initial desire to void. It is therefore useful in the treatment and prevention of both incontinency and frequent voluntary urination. The efficacy of racemic oxybutynin in the bladder has been attributed to a combination of antimuscarinic, direct spasmolytic and local anesthetic effects on the detrusor smooth muscle. Because of the antimuscarinic activity of the racemic drug, xerostomia (dry mouth) and mydriasis (dilated pupils), which involve muscarinic cholinergic receptors, are very common side effects. In fact, at least one researcher has referred to the "inevitable symptoms of mydriasis, xerostomia, tachycardia, etc." that accompany the administration of racemic oxybutynin [Lish et al. Arch. Int. Pharmacodyn. 156, 467-488 (1965), 481]. The high incidence of anticholinergic side effects (40 to 80%) often results in dosage reduction or discontinuation of therapy.

Pharmacological studies of the individual enantiomers have suggested that the R-enantiomer is the efficacious enantiomer. Noronha-Blob et al. [J. Pharmacol. Exp. Ther. 256, 562-567 (1991)] concluded that the cholinergic antagonism of racemic oxybutynin (measured in vitro by its affinity for M₁, M₂ and M₃ receptors subtypes and in vivo for diverse physiological responses) could be attributed mainly to the activity of the R-enantiomer. For all responses they found the rank order of potency of racemic oxybutynin and its enantiomers to be the same, namely, (R)-oxybutynin greater than or equal to racemic oxybutynin, which was much greater than (S)-oxybutynin, with (S)-oxybutynin being 1 to 2 orders of magnitude less potent than (R)-oxybutynin.

SUMMARY OF THE INVENTION

It has now been unexpectedly found that the substantially optically pure S enantiomer provides a superior therapy for the treatment of urinary incontinence.

Optically pure (S)-oxybutynin provides this treatment while substantially reducing the adverse effects that primarily arise from anticholinergic activity and that are associated with the administration of racemic oxybutynin. These include, but are not limited to, xerostomia, mydriasis, drowsiness, nausea, constipation, palpitations and tachycardia. The amelioration of cardiovascular side effects of racemic oxybutynin, such as tachycardia and palpitations, by the administration of (S)-oxybutynin is of particular therapeutic value.

The active compound of these compositions and methods is an optical isomer of oxybutynin. The preparation of racemic oxybutynin is described in British Patent Specification 940,540. Chemically, the active compound is the S enantiomer of 4-(diethylamino)-2-butynyl α-cyclohexyl-α-hydroxybenzeneacetate also known as 4-(diethylamino)-2-butynyl phenylcyclohexylglycolate, and hereinafter referred to as oxybutynin. The generic name given to the hydrochloride salt of racemic oxybutynin by the USAN Council is oxybutynin chloride; it is sold under the trade name of Ditropan®. The isomer of oxybutynin having the S absolute stereochemistry (Registry Number 119618-22-3) is dextrorotatory, and is shown in Formula I: ##STR1##

The synthesis of (S)-oxybutynin has been described [Kachur et al. J. Pharmacol. Exp. Ther. 247, 867-872 (1988)], but (S)-oxybutynin itself is not presently commercially available. All of the clinical results that have been reported have been obtained with the racemic mixture, although the pharmacology of the individual enantiomers has been described in guinea pigs and rats [see Kachur et al. J. Pharmacol. Exp. Ther. 247, 867-872 (1988) and Noronha-Blob et al. J. Pharmacol. Exp. Ther. 256, 562-567 (1991)].

In one aspect the invention relates to a method for treating urinary incontinence while avoiding concomitant liability of adverse effects, which comprises administering to a human in need of such treatment a therapeutically effective amount of (S)-oxybutynin or a pharmaceutically acceptable salt thereof, substantially free of its R enantiomer. The term "substantially free of its R enantiomer" as used herein means that the compositions contain at least 90% by weight of (S)-oxybutynin and 10% by weight or less of (R)-oxybutynin.

In a more preferred embodiment, the composition contains at least 99% by weight of (S)-oxybutynin and 1% or less (R)-oxybutynin. The substantially optically pure (S)-oxybutynin may be administered parentally, rectally, intravesically, transdermally, orally or by aerosol, orally and transdermally being preferred, at a rate of about 1 mg to about 100 mg per day.

In another aspect, the invention relates to a pharmaceutical unit dosage form comprising a therapeutically effective amount of (S)-oxybutynin, or a pharmaceutically acceptable salt thereof, substantially free of its (R) stereoisomer, and a pharmaceutically acceptable carrier in the form of a tablet or transdermal delivery device. The tablet preferably contains from 0.5 to 25 mg of (S)-oxybutynin and is prepared by conventional methods, well-known in the art. The transdermal administration is improved by the inclusion of a permeation enhancer in the transdermal delivery device, for example as described in PCT application WO 92/20377.

DETAILED DESCRIPTION OF THE INVENTION

The S enantiomer of oxybutynin may be obtained by resolution of the intermediate mandelic acid followed by esterification as described by Kachur et al. (op. cit.). Alternatively, the S enantiomer may be obtained by the resolution of racemic oxybutynin using conventional means such as fractional crystallization of diastereomeric salts with chiral acids. Other standard methods of resolution known to those skilled in the art, including, but not limited to, simple crystallization and chromatography on a chiral substrate can also be used.

The magnitude of a prophylactic or therapeutic dose of (S)-oxybutynin in the acute or chronic management of disease will vary with the severity and nature of the condition to be treated and the route of administration. The dose and perhaps the dose frequency will also vary according to the age, body weight and response of the individual patient. In general, the total daily dose range for (S)-oxybutynin for the conditions described herein is from about 1 mg to about 100 mg in single or divided doses, preferably in divided doses. In managing the patient, the therapy should be initiated at a lower dose, perhaps at about 0.25 mg to about 25 mg, and increased up to about 100 mg depending on the patient's global response. It is further recommended that patients over 65 years and those with impaired renal or hepatic function initially receive low doses and that they be titrated based on individual response(s) and blood level(s). It may be necessary to use dosages outside these ranges in some cases, as will be apparent to those skilled in the art. Further, it is noted that the clinician or treating physician will know how and when to interrupt, adjust, or terminate therapy in conjunction with individual patient response. The terms "a therapeutically effective amount" and "an amount sufficient to treat incontinence but insufficient to cause adverse effects" are encompassed by the above-described dosage amounts and dose frequency schedule.

Any suitable route of administration may be employed for providing the patient with an effective dosage of (S)-oxybutynin. For example, oral, rectal, parenteral (subcutaneous, intramuscular, intravenous), transdermal, aerosol and like forms of administration may be employed. Additionally, the drug may be administered directly into the bladder through the urethra, as described for racemic oxybutynin by Massad et al. [J. Urol. 148, 595-597 (1992)]. Dosage forms include tablets, troches, dispersions, suspensions, solutions, capsules, transdermal delivery systems, and the like.

The pharmaceutical compositions of the present invention comprise (S)-oxybutynin as the active ingredient, or a pharmaceutically acceptable salt thereof, and may also contain a pharmaceutically acceptable carrier, and optionally, other therapeutic ingredients.

The terms "pharmaceutically acceptable salts" or "a pharmaceutically acceptable salt thereof" refer to salts prepared from pharmaceutically acceptable non-toxic acids. Suitable pharmaceutically acceptable acid addition salts for the compound of the present invention include acetic, benzenesulfonic (besylate), benzoic, camphorsulfonic, citric, ethanesulfonic, fumaric, gluconic, glutamic, hydrobromic, hydrochloric, isethionic, lactic, maleic, malic, mandelic, methanesulfonic, mucic, nitric, pamoic, pantothenic, phosphoric, succinic, sulfuric, tartaric, p-toluenesulfonic, and the like. The hydrochloride has particular utility and was, in fact, the salt used in the studies described below.

The compositions of the present invention include suspensions, solutions, elixirs, or solid dosage forms. Carriers such as starches, sugars, and microcrystalline cellulose, diluents, granulating agents, lubricants, binders, disintegrating agents, and the like are suitable in the case of oral solid preparations (such as powders, capsules, and tablets), and oral solid preparations are preferred over the oral liquid preparations.

Because of their ease of administration, tablets and capsules represent one of the more advantageous oral dosage unit forms, in which case solid pharmaceutical carriers are employed. If desired, tablets may be coated by standard aqueous or nonaqueous techniques.

In addition to the common dosage forms set out above, the compounds of the present invention may also be administered by controlled release means and delivery devices such as those described in U.S. Pat. Nos. 3,845,770; 3,916,899; 3,536,809; 3,598,123; and 4,008,719, and PCT application WO 92/20377, the disclosures of which are hereby incorporated by reference.

Pharmaceutical compositions of the present invention suitable for oral administration may be presented as discrete units such as capsules, cachets, or tablets, each containing a predetermined amount of the active ingredient, as a powder or granules, or as a solution or a suspension in an aqueous liquid, a non-aqueous liquid, an oil-in-water emulsion, or a water-in-oil liquid emulsion. Such compositions may be prepared by any of the methods of pharmacy, but all methods include the step of bringing into association the active ingredient with the carrier which constitutes one or more necessary ingredients. In general, the compositions are prepared by uniformly and intimately admixing the active ingredient with liquid carriers or finely divided solid carriers or both, and then, if necessary, shaping the product into the desired presentation, just as is known for the racemic mixture.

The surprising utility of the S enantiomer has been established by the following studies.

Binding of (R)- and (S)-Oxybutynin to Human M1, M2, M3 and M4 Muscarinic Receptors MATERIALS AND METHODS

Protein source

The experiments were carried out on membranes prepared from SF9 cells infected with baculovirus to express the human recombinant M₁, M₂, M₃ and M₄ muscarinic receptor subtypes.

Binding Assays

    __________________________________________________________________________                                     Reference                                      Receptor                                                                            Radioligand                                                                            Conc.                                                                              Nonspecific                                                                            Incubation                                                                            Compound                                       __________________________________________________________________________     M.sub.1H                                                                            [.sup.3 H]pirenzepine                                                                    2 nM                                                                             atropine (1 μM)                                                                     60 min/27° C.                                                                  pirenzepine                                    M.sub.2H                                                                            [.sup.3 H]AF-DX 384                                                                      2 nM                                                                             atropine (1 μM)                                                                     60 min/27° C.                                                                  methoctramine                                  M.sub.3H                                                                            [.sup.3 H]4-DAMP                                                                       0.8 nM                                                                             atropine (1 μM)                                                                     60 min/27° C.                                                                  4-DAMP                                         M.sub.4H                                                                            [.sup.3 H]4-DAMP                                                                       0.3 nM                                                                             atropine (1 μM)                                                                     60 min/27° C.                                                                  4-DAMP                                         __________________________________________________________________________

Following incubation, the assays were rapidly filtered under vacuum through GF/B glass fiber filters (Whatman) and washed with an ice-cold buffer using a Brandel Cell Harvester. Bound radioactivity was determined with a liquid scintillation counter (LS 6000, Beckman) using a liquid scintillation cocktail (Formula 99, DuPont NEN).

Experimental Protocol

The compounds were tested on each receptor at 10 concentrations in duplicate to obtain competition curves. In each experiment, the reference compound for the receptor under investigation was simultaneously tested at 8 concentrations in duplicate to obtain a competition curve in order to validate this experiment.

Analysis and expression of results

The specific radioligand binding of each receptor was defined as the difference between total binding and nonspecific binding determined in the presence of an excess of unlabelled ligand. IC₅₀ values (concentrations required to inhibit 50% of specific binding) were determined by non linear regression analysis of the competition curves. These parameters were obtained by curve fitting using Sigmaplot™ software.

                  TABLE 1                                                          ______________________________________                                         Binding of R-oxybutynin and S-oxybutynin to human                              muscarinic subtypes M1-M4                                                              R-OXY     S-OXY     Ref. Compound                                      Receptor                                                                               IC.sub.50 (nM)                                                                           IC.sub.50 (nM)                                                                           IC.sub.50 (nM)                                     ______________________________________                                         M1      0.99      47.6      Pirenzepine                                                                               11.9                                    M2      9.9       178       Methoctramine                                                                             14.6                                    M3      1.8       149       4-DAMP     1.6                                     M4      1.2       100       4-DAMP     0.87                                    ______________________________________                                    

Binding of (R)- and (S)-Oxybutynin to Calcium Channels MATERIALS AND METHODS

Binding assays

Binding assays were performed using the following methods:

    ______________________________________                                                                 Reference                                              Receptors     Membranes Compounds References                                   ______________________________________                                         Ca channel    rat cerebral                                                                             diltiazem Schoemaker                                   (T + L, diltiazem site)                                                                      cortex              and Langer                                                                     (1985)                                       Ca channel    rat cerebral                                                                             D600      Reynolds                                     (T + L, verapamil site)                                                                      cortex              et al (1986)                                 ______________________________________                                    

The experiment conditions were:

    __________________________________________________________________________     Receptors   Ligands                                                                               Concentrations                                                                         Nonspecific                                                                              Incubation                                __________________________________________________________________________     Ca channel  [.sup.3 H] diltiazem                                                                    5 nM  dilitiazem (10 μM)                                                                    120 min/25° C.                     (T + L, diltiazem site)                                                        Ca channel  [.sup.3 H]D 888                                                                       0.5 nM  D 600 (10 μM)                                                                          60 min/22° C.                     (T + L, verapamil site)                                                        __________________________________________________________________________

Following incubation, the assays were rapidly filtered under vacuum through GF/B or GF/C glass fiber filters (Whatman) and washed with an ice-cold buffer using a Brandel Cell Harvester. Bound radioactivity was determined with a liquid scintillation counter (LS6000, Beckman) using a liquid scintillation cocktail (Formula 989, DuPont NEN).

Experimental Protocols The compounds were tested in duplicate on each receptor at a concentration of 10⁻⁵ M. In each experiment, the reference compound for the receptor under investigation was simultaneously tested at 8 concentrations in duplicate to obtain a competition curve in order to validate this experiment.

Analysis and expression of results The specific radioligand binding of each receptor was defined as the difference between total binding and nonspecific binding determined in the presence of an excess of unlabelled ligand. Mean values, expressed as a percentage of inhibition of specific binding, are presented in Table 2. IC₅₀ values (concentration required to inhibit 50% of specific binding) were determined by non linear regression analysis of their competition curves. These parameters were obtained by curve fitting using Sigmaplot™ software.

                  TABLE 2                                                          ______________________________________                                         Binding of R-oxybutynin and S-oxybutynin                                       to calcium channels                                                            [Inhibition (in %) of diltiazem and verapmil binding                           to calcium channel receptors.]                                                          R-OXY     S-OXY     Ref. Compound                                     Receptor (10.sup.-5 M)                                                                            (10.sup.-5 M)                                                                            IC.sub.50 (nM)                                    ______________________________________                                         Calcium  86        59        diltiazem  55.8                                   (diltiazem)                                                                    Calcium  86        68        D600       36.4                                   (verapamil)                                                                    ______________________________________                                    

While it is well known that the normal emptying of the bladder is mediated through cholinergic mechanisms, the bladder instability that is seen in patients suffering from incontinence appears to be related to non-cholinergic contractions of the bladder. Andersson et al. [Neurourol Urodyn 5, 579-586 (1986)] have shown in animals that the atropine-resistant detrusor muscle is highly sensitive to calcium antagonists.

The study of the receptor binding affinity of (R)- and (S)-oxybutynin to the receptor sites for the calcium channel blockers diltiazem and verapamil described above allows one to conclude that S-oxybutynin has therapeutic effects on involuntary micturition, while this isomer (contrary to the R-isomer and the racemate) has very little effect on the normal voiding mechanism and also has significantly decreased anticholinergic side effects as compared with the R-isomer and racemate. The avoidance of cardiovascular side effects that arise from the anticholinergic action of racemic oxybutynin is of particular note. 

We claim:
 1. A method for treating urinary incontinence while avoiding concomitant liability of adverse effects, which comprises administering to a human in need of such treatment a therapeutically effective amount of (S)-oxybutynin or a pharmaceutically acceptable salt thereof, substantially free of its (R) enantiomer.
 2. The method of claim 1 wherein (S)-oxybutynin is administered by inhalation or by parenteral, transdermal, rectal or oral administration.
 3. The method of claim 2 wherein the amount of (S)-oxybutynin or a pharmaceutically acceptable salt thereof administered is from about 1 mg to about 100 mg per day.
 4. The method according to claim 1 wherein (S)-oxybutynin, or pharmaceutically acceptable salt thereof, is administered orally.
 5. The method according to claim 1 wherein (S)-oxybutynin, or pharmaceutically acceptable salt thereof, is administered transdermally.
 6. A pharmaceutical unit dosage form which comprises a therapeutically effective amount of (S)-oxybutynin, or a pharmaceutically acceptable salt thereof, substantially free of its (R) stereoisomer, and a pharmaceutically acceptable carrier in the form of a tablet.
 7. A pharmaceutical unit dosage form according to claim 6 comprising from 0.5 to 25 mg of (S)-oxybutynin.
 8. A pharmaceutical unit dosage form which comprises a therapeutically effective amount of (S)-oxybutynin, or a pharmaceutically acceptable salt thereof, substantially free of its (R) stereoisomer, and a pharmaceutically acceptable carrier in the form of a transdermal delivery device.
 9. A pharmaceutical unit dosage form according to claim 8 wherein said pharmaceutically acceptable carrier comprises a permeation enhancer. 